1. Field of the Invention
The present invention relates to backlight driving circuit and liquid crystal display having the same.
2. Description of Prior Art
LCDs are commonly used as displays for compact electronic apparatuses, because they not only provide good quality images but are also very thin. The liquid crystal in an LCD does not emit any light itself. The liquid crystal has to be lit by a light source so as to clearly and sharply display text and images. Thus, a backlight module and a backlight driving circuit for driving the backlight module are generally needed for an LCD.
Referring to FIG. 3, a typical backlight driving circuit is shown. The backlight driving circuit 10 includes a first transistor 140, a second transistor 150, and a first reverser 120, a second reverser 130, an AND gate 110, a first power supply 160, a second power supply 170, and a signal output terminal 180. The first power supply 160 provides a 18V voltage, and the second power supply 170 provides a 11V voltage.
The AND gate 110 has a first input end 111, a second input end 112, and an output end 113. The first input end 111 is the first signal input end of the backlight driving circuit 10, and the second input end 112 is the second signal input end of the backlight driving circuit 10.
The first transistor 140 is a P-Channel enhancement mode metal-oxide-semiconductor field-effect transistor (P-MOSFET), having a gate electrode 141 connected to the output end 113 of the AND gate 110 through the first reverser 120, a source electrode 142 connected to the first power supply 160, a drain electrode 143 connected to the signal output terminal 180.
The second transistor 150 is a N-Channel enhancement mode metal-oxide-semiconductor field-effect transistor (N-MOSFET), having a gate electrode 151 connected to the output end 113 of the AND gate 110 through the second reverser 130, a source electrode 152 connected to the ground, a drain electrode 153 connected to the signal output terminal 180.
The first reverser 120 has a first positive power supply terminal 121 and a first negative power supply terminal 122. The first positive power supply terminal 121 is connected to the first power supply 160, and the first negative power supply terminal 122 is connected to the ground. The second reverser 130 has a second positive power supply terminal 131 and a second negative power supply terminal 132. The second positive power supply terminal 131 is connected to the second power supply 170, and the second negative power supply terminal 132 is connected to the ground.
In use, when the first input end 111 of the AND gate 110 receives a low-level voltage signal, the AND gate 110 outputs a low-level signal, and the second reverser 130 outputs a high-level signal, which turns on the second transistor 150. Thus, the signal output terminal 180 is connected to the ground through the drain electrode 153 and the source electrode 152 of the second transistor 150. Therefore, the backlight driving circuit 10 can not effectively work.
When the first input end 111 of the AND gate 110 receives a high-level signal, the output signal of the AND gate 111 changes following the change of the input signal of the second input end 112. If the input signal of the second input end 112 is invariable, one of the first and the second transistors 140, 150 keeps on-state. If the first transistor 140 turns on all along, the signal output terminal 180 outputs a 18V high-level voltage. If the second transistor 150 turns on all along, the signal output terminal 180 is connected to the ground through the second transistor 150. Thus, the signal output terminal 180 outputs the high-level voltage all the time or is connected to the ground all the time. Therefore, the backlight driving circuit 10 can not effectively work.
When the first input end 111 of the AND gate 110 receives a high-level voltage signal, and the second input end 112 receives a pulse signal, the first and the second transistors 140, 150 turn on alternately. Thus, the signal output terminal 180 alternately outputs high-level and low-level signals. The backlight driving circuit 10 can realize effective works.
When the second input end 112 receives a low-level signal, the AND gate 110 outputs a low-level voltage signal. The first and the second reversers 120, 130 respectively output 18V high-level voltage signal and 11V low-level voltage signal. Thus, the first transistor 140 turns off, and the second transistor 150 turns on. The signal output terminal 180 is connected to the ground through the second transistor 150 and outputs a 0V low-level voltage signal.
When the second input end 112 receives a high-level signal, the AND gate 110 outputs a high-level voltage signal. The first and the second reversers 120, 130 all output 0V low-level voltage signal. Thus, the first transistor 140 turns on, and the second transistor 150 turns off. The signal output terminal 180 is connected to the first power supply 160 through the first transistor 150 and outputs a 18V high-level voltage signal.
Therefore, when the first input end 111 receives a high-level signal, and the second input end 112 receives a pulse signal, the backlight driving circuit 10 can periodically output high-level and low level voltage signals.
However, the first transistor 120 is a P-MOSFET, which has a higher on resistance, generally being 0.1 ohm. The higher on resistance makes the first transistor 120 to consume a larger power and produce a larger heat energy, which heats the first transistor 140, and influences the operation efficiency of the first transistor 140. Therefore, the stability of the backlight driving circuit 10 is decreased. In addition, the P-MOSFET has a high cost, which also adds the cost of the backlight driving circuit 10.
It is desired to provide a transflective mode liquid crystal display that can solve the above-mentioned brightness and color problems.